1. Field of the Invention
The present invention relates to a control method for an antiskid braking system for preventing wheels of an automobile from being locked when the automobile is braked, and more particularly, to a control method for an antiskid braking system adapted for use in a four-wheel-drive automobile.
2. Description of the Related Art
Automobiles with an antiskid braking system have recently been increasing. The antiskid braking system can prevent wheels of an automobile from skidding or locking when the automobile is braked. Thus, automobiles with this system can enjoy improved driving stability and greatly reduced braking distance when they are braked.
According to the antiskid braking system, the respective speeds of the individual wheels, that is, wheel speeds, are obtained by means of wheel speed sensors, and a pseudo speed of the body of the automobile or a reference body speed is calculated on the basis of these wheel speeds. If any one of the wheel speeds is lower than the calculated reference body speed by a predetermined value out of the permissible range, it is concluded that the wheel concerned is subject to a skid. In this case, the system controls the brake unit associated with this wheel in order to reduce the braking force on the wheel. When the braking force on the wheel is reduced so that the wheel speed is restored, thereafter, the system repeatedly hold, increase, or reduce the braking force as required by activating the brake unit, thereby causing the wheel speed to follow up the reference body speed. Thus, the wheel can be effectively prevented from locking.
In order to optimally control the braking force on a wheel which tends to skid by means of the antiskid braking system, the reference body speed should be accurately calculated, as is evident from the above description. According to the conventional system, a specific wheel speed, among other wheel speeds, is selected as a criterion wheel speed, and the reference body speed is set on the basis of this criterion wheel speed.
According to the aforesaid method for calculating the reference body speed, the reference body speed can be accurately obtained without hindrance for all the driving conditions of the automobile if the automobile is of the two-wheel-drive type. In the case of a four-wheel-drive automobile, however, the calculated reference body speed takes an inaccurate value in a certain driving condition.
The driving condition in which the reference body speed is inaccurate is a case where all the wheels of the four-wheel-drive automobile slip or skid simultaneously. Since the rotational behavior of front and rear wheel of the automobile of this type are restricted to each other by means of a drive system which connects between the front and rear wheel sides, the four wheels are liable to skid simultaneously.
If the specific wheel speed is selected as the criterion wheel speed, that is, the reference body speed, when all the four wheels are skidding, the reference body speed naturally takes a smaller value than that of the actual body speed. In this case, however, the deviation between the reference body speed and each wheel speed takes only a small value. In these circumstances, therefore, the antiskid braking system concludes that the road on which the automobile is running is a high-.mu. road, and further increases the braking force on each wheel, thereby possibly helping all the four wheels lock at the same time. The reference .mu. indicates the coefficient of friction between the road and the wheel.
To cope with this, the antiskid braking system may be provided with a sensor for detecting the deceleration of the body. With use of this sensor, either the criterion wheel speed obtained from the wheel speed sensors or an output from the deceleration sensor can be selected so as to calculate the reference body speed, depending on the driving conditions.
The deceleration sensor, however, cannot deliver a signal indicative of the deceleration immediately after it starts to receive the deceleration, due to the detecting mechanism of the sensor for the deceleration. In the initial stage of the automobile braking operation, therefore, the output from the deceleration sensor is subject to a response delay. Thus, a reference body speed of a desired accuracy cannot be obtained by only selecting and setting the output of the deceleration sensor or the criterion wheel speed as the reference body speed.